This invention relates to overhead projectors and in particular to an overhead projector which is provided with a projection lens that is movable parallel to the plane of a stage on which an original is placed, and by this movement of the projection lens trapezoidal distortion of a projective image and loss of focus can be eliminated when projecting an original at an oblique angle to a screen at a vertical position.
This type of overhead projector is disclosed in Examined Japanese Patent Publication No. 55-18891 and U.S. Pat. No. 3,547,530 (Patented Dec. 15, 1970).
As illustrated in FIG. 1, when projecting an image to a vertical screen a at an oblique angle .theta., trapezoidal distortion and loss of focus occure on the projected image b of the screen. In order not to cause such trapezoidal distortion and loss of focus, a device is made to move a projection lens c and mirror g in the direction parallel to the plane of stage d.
By the way, in this type of overhead projector, it is required, in order not to lower the illumination efficiency, that the illumination light transmitted to illuminate an original through an illuminating system should always be collected at the position of an entrance pupil of the projection lens irrespective of the movement of the projection lens c.
In the said prior art, such requirement is satisfied by tilting a Fresnel lens h or by moving it parallel to the plane of a stage d on which an original is placed. However, this entails the problem that the construction of the device becomes complicated and the device itself becomes larger in size in order to move a larger and heavier Fresnel lens h alone or together with the stage d. Particularly, the type of device wherein the Fresnel lens is tilted becomes more complicated in construction since a light source e is also moved. Further, a simple and quick change of the lamp can not be hoped for because the light source also moves.